Method for tin plating



Patented Aug. 20,1946

2,408,180 'Mn'rnon Foa m mmo Nowell r. Blackburn, Niagara Falls, N. Y., assignor to E. I. du Pont de Nemours & Company, I Wilmington, Del., a corporation of Delaware No Drawing. Application May 20, 1943,- Serial No. 487,813

7 Claims.

This invention relates to the electroplating of metals and particularly to the electrodeposition of tin from alkali metal stannate electroplating baths.

In present commercial practice for the electroplating of tin, alkaline plating baths containing sodium stannataor potassium stannate are commonly used. Although sodium stannate electroplating baths may be operatedsatisfactorily at comparatively low current densities, such baths have the disadvantage that when it is desired to operate at high current densities in order to obtain high plating speed the anode efilciency becomes so low as to make such baths unsatisfactory for commercial operation. Although sodium stannate electroplating baths have been successfully used at current densities up to about 50 A/SF, no method has been available hitherto by which these electroplating baths could be satisfactorily operated at current densities substantially above 50 A/SF. In the case of potassium stannate electroplating baths the difllculty encountered in obtaining satisfactory anode efficiencies exists at even lower current densities and lack of a method for operating such baths at high current densities has seriously hampered the commercial development of the potassium stannate bath. Heretofore no method has been available which would permit operation of stannate electroplating baths at high current densities with satisfactory anode efficiency, and it has therefore been impossible to fully utilize their potential capacity.

It is one of the objects of this invention to provide a new and improved method for alkaline tin plating. Another object is to provide an electroplating bath for the electrodepositlon of tin which will operate efllciently at high current densities. A further object is to provide a novel anode for alkali metal stannate electroplating baths. These and other objects will be apparent from the ensuing description of my invention.

The above objects are attained in accordance with my invention by electroplating tin from an alkali metal stannate plating bath having an anode or anodes comprising an alloy of tin and one or more alkali metals. I have discovered that when a tin anode containing a relatively small amount of alkali metal is utilized in the stannate plating bath, a surprising and unexpected increase in anode efficiency is obtained and 'stannate plating baths utilizing these novel 2 anodes may be operated at high current densities with high electrode efiiciencies.

The novel alloy anodes of'my invention may be prepared by mixing molten tin with a molten alkali metal and molding the mixture in the desired form. For example, pure tin is heated in an iron or silica crucible to approximately 270 C. The required weightof alkali metal is then added to the molten tin. It may be desirable to utilize about 0.1 to 0.2% excess alkali metal to compensate for possible loss through oxidation during the mixing and molding operation. .The

alkali metal is introduced under the surface of the molten tin, for example, by introducing the alkali metal under an inverted crucible held in position under the surface of the molten tin. The mixture is agitated until all lumps have disappeared, the surface skimmed to remove oxidation products, and the melt poured into a mold and allowed to cool.

In general, the methods commonly used for the preparation of alkali metal alloys may be utilized in the preparation of my novel anodes. Preferably the molten mixture is protected by an atmosphere of inert gas such as nitrogen in order to avoid oxidation. The molten tin should be heated to a temperature somewhat above the melting point of the alloy to be formed.

The concentration of alkali metal present in the novel anodes of my invention may be varied over a considerable range with satisfactory results. Usually only relatively small amounts of alkali metal are required, 1. e. 0.1 to 10% by weight. I have found that amounts of sodium up to 10% or of potassium up to 5% of the weight of the alloy are sufficient for practical operation. When concentrations of alkali metal greater than 10% by weight are utilized the solubility of the anode in the plating solution tends to become too high for satisfactory operation. Furthermore, substantially higher concentrations of alkali metal may decrease the mechanical strength of the anode below practical limits. The particular concentration of alkali metal to be used in any given plating bath will depend upon the current density at which it is desired to operate. At low current densities concentrations of alkali metal as low as 0.1% may be sufflcient to effect the required anode efficiency. When it is desired to operate at high current densities, for example, current densities of to A/SF, concentrations of alkali metal as high in the operation of a stannate plating bath con-' taining 135 g./l. of potassium stannate and 21 g./1. or potassium hydroxide at 90 C. and 125 A/SF, an anode efliciency of 90% is obtained when the concentration of potassium in the tin anode is 3.8%; and when the same bath is operated at 150 A/SF using the same anode an anode efilciency of 80% is obtained.

The following tables illustrate the high anode efliciencies obtained by the use or my novel anode. In obtaining these data sodium stannate baths contained 120 g./l. of sodium stannate and 15 g./l. of sodium hydroxide were used and the potassium stannate baths contained 135 g./l. of potassium stannate and 21 g./1. of potassium hydroxide. These materials were dissolved in distilled water and the plating baths were maintained at 90 C. i

In the tables and throughout the specification the term "anode efllciency" refers to the anode efllciency based on tetravalent tin.

Table 1 represents a comparison of the anode eillciencies obtained with a pure tin anode and with sodium-tin alloy anodes over a current density range of 35 to 150 A/SF in a sodium stannate plating bath. Table 2 represents a similar comparison of potassium-tin anodes with pure tin anodes in a potassium stannate plating bath. Table 3 illustrates a comparison of the results obtained in using a potassium-tin anode and a pure tin anode in a sodium stannate plating bath. Table 4 shows a comparison of a sodium-tin anode with a pure tin anode in a potassium stannate plating bath.

TABLE 1 Axons EmcIENcY Comrmusorz Sodium-tin alloy anode vs. tin anode in sodium stannate plating bath Current density in A/BF Percent sodium 50 75 100 Percent efllcicncy TABLE 2 Potassium-tin alloy anode vs. tin anode in potassium stannate plating bath Current density in A/SF Percent potassium manode 2s 60 75 100 125 150 200 1 ercent efllciency 4 new a 1.2% potassium-tin alloy anode vs. tin anode m a sodium stannate plating bath Percent emcienoy Current dentisy in AIBF 1 27 potaseiufii alloy 3235 anode TABLE 4 0.75% sodium-tin alloy anode vs; tin anode, in a potassium stannate plating bath Percent eflloiency Current density in A/SF s i ii pm alloy anode mods 97 80 B8 64 i 82 4o 70 E 100 53 I0 placed in series with a silver coulometer for measuring the current used. An accurate ammeter was connected in series and a volt meter was connected across the bus bars of the plating bath. The temperature 01 the plating bath was maintained constant throughout the operation.

The anodes used were cylindrical and had a diameter 01' /2", a length of 3 A" and an area of approximately 5% square inches. The anodes were placed in the bath between two cathodes having a total area equal tothat of the anode. The anodes were polarized before drying and weighing and the coulometer cathode was air dried and weighed. The baths were operated suiliclently long to deposit from 2 to 3 grams oi silver on the coulometer in each efllciency run. Several runs were made and average values taken. Anodes were kept in a polarized condition at all times by increasing the current density until gasing was observed. Anode efficiencies were calculated by means of the following formula:

Effi ciency where A=anode loss and C=coulometer gain.

0.275=- g electroequivalent ratio As illustrated in the foregoing tables, the novel alloy anodes of my invention are especially effective in electroplating baths operated at high current densities. However, these anodes are also effective in increasing efficiency when baths are operated at lower current densities insince sodium is relatively inexpensive and is readily obtainable.

Although I have described my invention with particular reference to electroplating baths containing sodium stannate or potassium stannate utilizing alloy anodes comprising tin alloyed with sodium or potassium or both, it is to be understood that other alkali metals may be utilized in the novel anode of my invention, for example, lithium or caesium, and these metals may be alloyed with an anode containing tin with or without other alkali metals. Likewise, alkali metal stannate baths other than sodium and potassium somewhat. low, the use of alkali metal alloy' anodes makes it possible to obtain increased anode eificiency without adversely effecting cathode efiiciency. These and other advantages will be readily apparent to those skilled in the art.

I claim:

1. A process for electroplating tin which com- I prises electrolyzing an aqueous solution of an alkali metal stannate and an alkali metal hydroxide with an alloy anode comprising essentially tin and at least one alkali metal, the total alkali r metal content of said anode being 0.1 to 10% by weight.

2. A process for electroplating tin which comprises electrolyzing an aqueous solution of potassium stannate and potassium hydroxide with an alloy anode comprising essentially tin and at least one alkali metal, the total alkali metal content of said anode being 0.1 to 10% by weight.

3. A process for electroplatin tin which comprises electrolyzingan aqueous solution of sodium stannate and sodium hydroxide with an alloy anode comprising essentially tin and at least one alkali metal, the total-alkali metal content of said anode being 0.1 to 10% by weight.

4. A process for electroplating tin which comprises electrolyzing an aqueous solution of an alkali metal stannate and an alkali metal hydroxide with an alloy anode comprising essentially tin and 0.1 to 5% by weight of potassium.

5., A process for electroplating tin which comprises electrolyzing an aqueous solution of an alkali metal stannate and an alkali metal hydroxide with an alloy anode comprising essentially tin and 0.1 to 10% by weight of sodium.

6. A process for electroplating tin which comprises electrolyzing an aqueous solution of potassium stannate and potassium hydroxide with an alloy anode comprising essentially tin and 0.1 to 5% by weight of potassium.

7. A process for electroplating tin which comprises electrolyzing an aqueous solution of potassium stannate and potassium hydroxide with an alloy anode comprising essentially tin and 0.1 to 10% by weight of sodium.

" NEWELL F. BLACKBURN. 

